1. Field of the Invention
The present invention relates to a mobile communications system, a method of controlling the transmission power in the system, and a mobile station in the system, and more particularly, to a mobile communications system using code-division multiple access (CDMA), a method of controlling the transmission power in the system, and a mobile station for use in the system.
2. Description of the Related Art
CDMA is a method in which data is transmitted in the same frequency band and at the same time and is divided into parts in accordance with the spectrum-spread codes allocated to users. If CDMA is employed in a mobile communications system, the base station and mobile stations share the same frequency band, and the mobile stations have different spread codes allocated to them, respectively. Thus, communication can be achieved between the mobile stations.
The signal that the base station receives via the uplink extending from any mobile station to the base station is a composite signal that consists of the signals transmitted from all mobile stations existing in a specific area. In CDMA, the composite signal is subjecting to despreading that uses spread codes unique to the respective mobile stations, thereby dividing the composite signal into signals transmitted from the respective mobile stations as an output of a correlator of a receiver provided in the base station.
In CDMA using the direct spread method, however, the cross-correlation value for the spread codes to be allocated to any mobile station cannot be zero (0). Inevitably, the cross-correlation value may increase due to the influence that a signal from a mobile station near the base station imposes on a weak signal from a mobile station remote from the base station. In this case, the correlator of the receiver provided in the base station is going to be in synchronization with a signal of large cross-correlation value, because the receiver must receive signals from remote stations. Consequently, no data communication is possible between the base station and any remote station. This fault resulting from the distance between the base station and any remote mobile station is known as “near-far problem.”
To solve the near-far problem, a power-controlling method is performed to control the transmission power in each mobile station. That is, the base station generates control signals for controlling the transmission powers of the mobile stations, respectively. The control signals are transmitted to the mobile stations, so that the mobile stations may receive signals at the same level, no matter whether they are close or remote to the base station.
The control of transmission power in the uplink is a closed-loop transmission power control. In this control, the receipt SIR (Signal-to-Interference Ratio) concerning any mobile station is measured in the base station. The receipt SIR is compared with the target SIR (T-SIR). If SIR is smaller than the target SIR, the base station transmits an up-command to the mobile station as a transmission power control command (TPC command). If SIR is equal to or lager than the target SIR, the base station transmits a down-command to the mobile station as a TPC command. In accordance with the up-command or down-command, the closed-loop transmission power control is performed in the mobile station to control the transmission power.
SIR is the ratio of the interference wave to the desired signal. For the signal that the base station receives from a mobile station M(i), SIR is the ratio between the desired signal S(i) obtained through the despreading using the spread code of the mobile station M(i) and the sum of the interference waves obtained by inversely spreading the spread spectra from the other mobile stations by applying the spread code of the mobile station M(i). Even if SIR remains unchanged, the signal received may change in reception quality, depending on the transmission environment factors, such as number of transmission paths, the moving speed of the mobile station. Hence, the target SIR, i.e., reference SIR, is corrected at long intervals, in accordance with the quality of the signal received, which is measured for a relatively long period. That is, the base station increases T-SIR for each mobile station if the quality of the signal received from the mobile station is poor, and decreases T-SIR if the quality of the signal received is no longer poor and stable for a prescribed period. Since T-SIR is thus controlled, data can be transmitted at minimum power and the base station can have a large signal-receiving capacity.
JP-A-2000-244391 discloses such a method of controlling transmission power as described above in conjunction with the prior art (see FIGS. 29 and 32). To facilitate the understanding of the present invention, the prior art will be described in detail, with reference to the method disclosed in the publication.
FIGS. 9 and 10 show the base station and one of mobile stations, respectively, which are provided in a CDMA mobile communications system that is disclosed, as prior art, in the above-identified publication. As shown in FIG. 9, the base station BS receives radio waves transmitted from the mobile stations MSa to MSn that exist in an area. In the base station BS, a receiving radio module 212 demodulates a base-band signal and performs a receiving process at high/intermediate frequencies. Synchronous acquisition/despreading circuits 213a to 213n perform synchronous acquisition and despreading, using the spread codes allocated to the respective mobile stations MSa to MSn. An uplink-channel SIR measuring unit 221 measures the SIRs of the signals transmitted from the mobile stations. An uplink-channel transmission-power control signal generating unit 222 compares the SIR of each mobile station with T-SIR (reference SIR) thereof, generating transmission-power control signals TPCa to TPCn for the respective mobile stations. Frame generating units 225a to 225n generates frames from the data signals encoded by encoding units 222a to 222n. Spreading circuits 223a to 223n spread the frames, by using the spread codes unique to the respective mobile stations. An adder circuit 226 adds the frames, generating a composite signal. The composite signal is modulated, frequency-converted and amplified by a transmitting radio module 224. The composite signal thus processed is transmitted from the antenna 210 to the mobile stations. Each transmission-power control signal is a signal that instructs the mobile station to decrease the transmission power at TPCi=0, if SIRi is equal to or larger than T-SIRi, and increase the transmission power at TPC =1, if SIRi is smaller than T-SIRi.
In the mobile station MSi shown in FIG. 10, a receiving radio module 12 receives radio waves from the base station, demodulates a base-band signal and performs a receiving process at high/intermediate frequencies. A synchronous acquisition/despreading circuit 13 performs synchronous acquisition and despreading on a signal, in which the spectra of a plurality of channels overlap one another, by using the spread code allocated to the mobile station. A detecting unit 14 carries out a wave-detecting process such as compensation for phase rotation. A decoding unit 15 performs an error-controlling process. The data obtained is used as received data. A transmission-power control signal received at the detecting unit 14 is input to a transmission-power control signal determining unit 40. The unit 40 determines whether the control signal is “1” or “0.” If the control signal is “1,” a selector 41 selects +1 db. If the control signal is “0,” the selector 41 selects −1 db. The selector 41 generates a control signal that represents a value by which the transmission power should be changed. This control signal is output to a transmission-power calculating unit 19. The unit 19 calculates an updated transmission power from the value and the transmission power now held in a transmission-power holding circuit 20. Meanwhile, an encoding unit 22 performs a correction-control process on the data to be transmitted. A frame-generating unit 25 generates a frame of a predetermined format. A spreading circuit 23 performs a spreading process, using the spread code unique to the mobile station. The resultant data is supplied to a variable amplifier 21. The variable amplifier 21 amplifies the signal to be transmitted, at such an appropriate gain that the data may be transmitted at the transmission power thus designated.
The control of the transmission power as described above enables the mobile station to transmit data at appropriate power in accordance with the transmission-power control signal, if the mobile station receives the transmission-power control signal from the base station without errors. In the CDMA mobile communications system, however, reception errors may occur in the downlink even if the reception electric field is relatively intense, inevitably because interference waves influence the mobile stations, too. In this case, the transmission-power control signal sent from the base station is demodulated by error. When this erroneous control signal controls the transmission power of the transmitter provided in the mobile station, the CDMA mobile communications system is no longer stable. This may reduce the signal-receiving capacity of the system or may interrupt the operation of the system.
The mobile station may receive a transmission-power control signal that is erroneous, causing the mobile station to transmit a signal at an increased power, though the data should be transmitted at a reduced power. If this is the case, the signal is inevitably amplified to a higher level and transmitted from the mobile station. In the base station, which receives radio waves from the other mobile stations, too, the interference-wave power increases with respect to the signals received from the other mobile stations. Consequently, the signals received from the other mobile stations are degraded in quality, possibly resulting in an interruption of data communication. Further, the increase in interference-wave power may decrease the number of mobile stations that can be incorporated in the system.
Conversely, the transmission-power control signal that the mobile station has received is erroneous, causing the mobile station to transmit a signal at a decreased power, though the data should be transmitted at an increased power. In the base station, which receives radio waves from the other mobile stations, too, the signal received from the mobile station falls in level with respect to the interference-wave power from the other mobile stations. Consequently, the signal received from the mobile station is degraded in quality, possibly resulting in an interruption of data communication.